The proposed research consists of several projects designed to elucidate the chemical nature and biological function of Fc receptors on white cells. 1. Antisera to isolated Fc receptors for IgG and IgE of lymphocytes and IgA Fc receptors of neutrophils will be prepared. 2. The effect of Fab anti-receptor antibody fragments on polyvalent B cell activation in vitro will be analyzed. 3. The NH2-terminal sequence of the isolated Fc receptor for IgE of cultured human lymphoblastoid cells will be determined by radioactive microsequencing. 4. The mechanism of activation of K lymphocytes by isolated Fc fragments of IgG2 and IgG4 will be investigated. It will be investigated if monocytes also become stimulated to kill target cells by Fc fragments of IgG2/4.